Radiation detecting element and radiation detecting device

ABSTRACT

There has been such a problem that radiation detecting elements using semiconductor elements have a low radiation detection efficiency, since the radiation detecting elements easily transmit radiation, even though the radiation detecting elements have merits, such as small dimensions and light weight. Disclosed are a radiation detecting element and a radiation detecting device, wherein a film formed of a metal, such as tungsten, is formed on the radiation incident surface of the radiation detecting element, and the incident energy of the radiation is attenuated. The efficiency of generating carriers by way of radiation incidence is improved by attenuating the incident energy, the thickness of the metal film is optimized, and the radiation detection efficiency is improved.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 13/321,246 filedon Feb. 14, 2012, which is a National Stage of PCT/JP2010/058295 filedon May 17, 2010, which claims foreign priority to Japanese ApplicationNo. 2009-120963 filed on May 19, 2009. The entire contents of each ofthe above applications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates a radiation detecting element, an imagediagnosis device and an imaging device utilized in the field of nuclearmedicine, atomic energy, astronomy, and cosmic-ray physics.

BACKGROUND ART

Positron Emission Tomography (PET) is one of the nuclear medicalinspection methods which measure the existence and position of theradioactivity. While the X-Ray CT measures the transmission of radiationby arranging radiation source out of analyte (human body), the PETmeasures the place of the radiation isotope (RI) distributed in specificinternal organs within the human body by administratingradiopharmaceuticals and measuring emitted radiation. The PET utilizesthe radiopharmaceuticals tagged by the RI, which releases positrons, asthe radiation source. The positron annihilates by combining withelectron bound near, and two strong gamma rays having strongtransmission power fly in opposite directions from that pointalternatively. By simultaneously counting a pair of radiations bydetectors arranged around the human body, direction and location of theradiation source can be detected. Pet is a technique to reconstructthree dimensional density of the radiation source inside where the datawere counted at the same time, for example cancer and generalexamination, brain, organs and various diagnostic like cardiac function,brain and nervous system research, and has been used innpharmacokinetics and metabolism research.

Semiconductor elements are used as radiation detecting element for PET,compared with radiation detecting elements for scintillator and aphotomultiplier tube that was conventionally used, it is small,lightweight, high image resolution characterized by direct radiationconverted to electrical signal, in recent year research has beendeveloped.

FIG. 7 (a) is a cross-sectional view of a radiation detecting elementusing a conventional Schottky diode disclosed in Patent Document 1. Onthe one surface of the CdTe-type semiconductor substrate 102, electrode101 comprised from InCdTe is formed. In the interface of the electrode101 and the semiconductor substrate 102, a Schottky junction is formed.On the other side of the semiconductor substrate 102, the ohmicelectrode 103 comprised from Pt is formed. When the bias voltage isapplied so that the high potential electrode 101 to the semiconductorsubstrate 102, a Schottky junction becomes reverse biased. In this case,electron-hole pairs are generated in the depletion layer in the Schottkyjunction and the radiation incident on the semiconductor substrate 102to move within the semiconductor substrate 102 by an electric fieldformed by the bias voltage. Incident radiation can be detected at highspeed by measuring the current flowing between electrode 101 and theohmic electrode 103.

PRIOR TECHNICAL DOCUMENTS PATENT DOCUMENTS

Patent Document 1: JP 2002-34400 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

About a conventional radiation detecting element using semiconductorelement shown in FIG. 7 (a), radiation detection efficiency is low, whencompared with radiation detecting elements with scintillator andphotomultiplier tubes. This cause has not been fully elucidated. Inparticular, if the radiation detecting element used in PET, method forsimultaneous detection with radiation detecting element disposed on bothsides of the analyte, gamma results are released in the oppositedirection, for example, For example, Sensitivity of one detector fallsto half, the sensitivity of PET system falls to ¼. Signal Noise Ratioworsening, detector devices hard to refinement thus increase theresolution.

Means Solving the Problems

The Invention (1) is a radiation detecting element comprised from anintrinsic semiconductor substrate, an electrode arranged in theradiation incident side on the intrinsic semiconductor substrate, and ann-type semiconductor region or a p-type semiconductor region arranged inthe opposite side, wherein the electrode is a metal film or a laminatedfilm including metal film.

The invention (2) is the radiation detecting element according to theinvention (1), wherein by defining x (cm) as the thickness of the metalfilm, a (1/cm) as the mass attenuation of the metal and b (cm) as themean range of the generated recoil electron in the metal, the thicknessx (cm) is arranged in the range of half-width of the approximatedradiation detection efficiency y=(1−exp(−a*x))*exp(−b*x) of theradiation detecting element.

The invention (3) is the radiation detecting element according to theinvention (1) or (2), wherein the radiation is gamma ray or X-ray.

Then invention (4) is the radiation detecting element of any one of theinventions (1) to (3), wherein the metal film the metal comprised fromW, Pt, In, Fe, Pb, Cu or these alloys.

The invention (5) is the radiation detecting element of any one of theinventions (1) to (3), wherein the metal film is the laminated filmcomprised from W/Pt, W/Fb, W/In, W/Cu or these alloys.

The invention (6) is the radiation detecting element of any one of theinventions (1) to (5), wherein the intrinsic semiconductor substrate ithe substrate comprised from Si, Ge, ZnO, CdTe, CdZnTe, SiC, GaN orGaAs.

The invention (7) is a radiation detecting device which detectsradiation by utilizing the radiation detecting element according to anyone of the inventions (1) to (6).

The invention (8) is a PET device which detects radiation by utilizingthe radiation detecting element according to any one of the inventions(1) to (6).

Effects of the Invention

1. By arranging the metal film to the radiation incident on theradiation detecting element so as to cause the incident radiation energyloss, the efficiency of carrier generation by incident radiation and theradiation detection efficiency can be improved.

2. Because the metal film of W, Pt, In, Fe, Pb, Cu or these alloys, orW/Pt, W/Pb, W/Fb, W/In, W/Cu or the laminated film of these alloy hashigh radiation blocking capability, and the radiation energy loss occursin the thin film, the radiation detecting element can be effectivelylightweighted and downsizing.

3. By utilizing the radiation detecting element with the intrinsicsemiconductor substrate, the thickness of the depletion layer can beformed in the crystal. Recoil electrons generated by incident radiationare generated through the layer in intrinsic semiconductor region anddepletion layer. When stop to move recoil electrons, generated manyholes and electrons, improve the response speed and sensitivity ofradiation detecting.

4. By utilizing the semiconductor detecting element having highradiation detection efficiency, it is possible to produce a radiationdetecting device like PET system etc. having high sensitivity and highresolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 (a) and (d) is a cross-sectional view of the radiation detectingelement of Schottky diode structure in accordance with the presentinvention.

FIG. 2 is a graph showing the dependence of the radiation detectionefficiency versus the thickness of the metal film.

FIG. 3 (a) is a schematic of the cross-sectional view of anotherembodiment of the radiation detecting element with Schottky diodestructure in accordance with the present invention. (b) is across-sectional view of radiation detecting element of PIN diodestructure in accordance with the present invention. (c) is across-sectional view of Radiation detecting element SIT structure inaccordance with the present invention.

FIG. 4 (a) is a plane view of the radiation detector array. (b) and (c)are cross-sectional view and external view of the radiation detectingdevice, respectively. (d) to (g) are external view of other embodimentsof the radiation detector array.

FIG. 5 is a block diagram of the radiation detection signal processingcircuit.

FIG. 6 (a) to (c) are examples of simulation results of the radiationdetecting sensitivity.

FIG. 7 (a) to (c) are cross-sectional views of the conventionalradiation detecting element.

EXPLANATION OF REFERENCE NUMERALS

-   -   1 electrode    -   2, 18 intrinsic semiconductor substrate    -   3, 17 N-type semiconductor layer    -   4, 16 ohmic electrode    -   5 gamma rays    -   6 electrons    -   11 ohmic electrode    -   12 N+ type semiconductor layer    -   13 semiconductor crystal    -   14 Schottky electrode    -   15 metal film    -   19 P-Type semiconductor layer    -   20 metal film    -   21 electrode    -   22 N+ type semiconductor layer    -   23 N-type semiconductor layer    -   24 P-type gate region    -   25 source electrode    -   26 insulating layer    -   27 shield film    -   31 crystals    -   32 radiation detecting element    -   33 electrode pad    -   34 bond wires    -   35, 36 radiation detector array    -   41 Radiation detecting diode    -   42 reset circuit    -   43 gamma rays detector    -   44 reset circuit unit    -   45 OR logic circuit    -   46 transfer circuit (Unit)    -   47 Computer    -   81, 85 crystal    -   82, 89 electrode    -   83 semiconductors    -   84, 87 Ohmic electrode    -   86 gamma rays    -   88 electrode stiffening plate    -   101 Schottky electrode    -   102 semiconductor crystal    -   103 Ohmic electrode    -   104 gamma rays

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, best mode of the present invention will be described.

<Radiation Detecting Element>

(Radiation Detecting Element Utilizing Semiconductor Element)

As a result of researching the cause of low sensitivity of radiationdetecting elements utilizing semiconductor elements, the inventor foundthat the radiation has strong transmission power to the semiconductor,and has high probability to transmit the semiconductor not formingcarriers. For example, in PET system, annihilation gamma rays, whichgenerate when positrons and electrons are combined and annihilate, havehigh energy 511 kev, and then have very strong transmission power. It isprofitable in the meaning that the ratio for the gamma rays generatedfrom the radiation isotope distributed in the human body to arrive atthe radiation detecting elements arranged out of the body is high. Butthere is a problem that it is difficult to achieve high sensitivedetection, because the gamma rays permit the semiconductor substrateeasily.

FIGS. 7 (b) and (c) show state when gamma rays are incident toconventional Radiation detecting elements. An electrode 101 comprisedfrom InCdTe has no high radiation blocking capability. Therefore, thegamma rays 104 incident to the electrode 101 (FIG. 7( b)) easily passthrough the electrode 101 and the semiconductor substrate 102 (FIG. 7(c)).

In order to solve this problem, an electrode comprised from metal like Wetc., which has high radiation blocking capability, was decided toarrange on the radiation incidence side of the detecting element. And,thickness of a Schottky electrode was decided to be set to optimumrange, in which the radiation detection efficiency is high. Also, anintrinsic semiconductor substrate was decided to be utilized as asemiconductor substrate.

FIG. 1 (a) is a cross-sectional view of a radiation detecting elementaccording to the present invention. About radiation detecting elementaccording to the present invention, for example, electrode 1 comprisedfrom W is arranged on the radiation incidence side of the intrinsicsemiconductor substrate 2 comprised from Si etc., and ohmic electrode 4is arranged on the opposite side. On the surface of the intrinsicsemiconductor substrate contacting to the ohmic electrode 4, N typesemiconductor layer 3 is formed by doping N type impurities like P(phosphorus) etc.

In the radiation detecting elements that are configured as describedabove, the direction from the electrode 1 to the intrinsic semiconductorsubstrate 2 becomes the forward direction of the diode. Then when thebias voltage from the outside power source is added so that the ohmicelectrode 4 has high electric potential compared to the electrode, aSchottky junction formed between the electrode 1 and intrinsicsemiconductor substrate 2 becomes reverse bias, and depletion layer isformed. Thus, when no radiation is incident, only microelectric currentlike leakage current flows in intrinsic semiconductor substrate 2. Also,when the radiation is entered, the number of the incident radiation canbe detected by measuring the pulse electric current generated accordingto the incident radiation.

Because the radiation detecting element according to the presentinvention has Schottky junction structure utilizing the intrinsicsemiconductor region, leak current is largely reduced compared to theconventional Schottky junction structure not utilizing the intrinsicsemiconductor region.

And, in the radiation detecting element of the present invention, metalfilm like W etc., in which radiation blocking capability is high, isutilized as the material of the electrode 1. It is different from theconventional radiation detecting element. Furthermore, the presentinventor found that radiation detection efficiency can be drasticallyraised compared to the conventional radiation detecting element bycontrolling the thickness of the metal film to an appropriate range. Itsmechanism has not yet been able fully understood. But it is conceivablethat the energy loss occurs when the radiation passes through the metalfilm, and electrons are excited by the energy generated there. Also itis conceivable that the incident radiation energy on the semiconductorsubstrate is attenuated in the metal film, and a probability thatcarriers generate by the radiation excitation in the semiconductorsubstrate becomes higher. if the metal film is too thin, radiationenergy loss or damping effect is not high. If the metal film is toothick, the number of radiation that can penetrate the metal film isremarkably reduced. On the other hand, if the thickness of the metalfilm is controlled to the adequate range according to the material, theefficiency of carrier generation by the incident radiation increases,and then detection efficiency is improved.

FIG. 1( b) to (d) are diagrams showing a state when the incident gammarays are incident on the radiation detecting element according to thepresent invention. When the gamma rays 5 incident on the electrode 1(FIG. 1( b)) passes through the electrode 1, its energy is attenuated.Being excited by incident gamma rays, carrier pairs consisting ofelectrons 6 and holes (not shown) are produced (FIG. 1( c)). A positivebias voltage is applied to the ohmic electrode 4, in comparison with theelectrode 1. By the electric field formed by the bias voltage, electronsare moved toward the ohmic electrode 4, and the holes are moved to oneside of the electrode. So a current flows in the intrinsic semiconductorsubstrate 2, and the incidence of radiation can be detected (FIG. 1(d)).

(Radiation Detection Efficiency Vs. the Thickness of the Metal Film)

FIG. 2 is the graph of the radiation detection efficiency vs. thethickness of the metal film by the radiation detecting element of thepresent invention. The detection efficiency is calculated using CernLibrary, Gate (GEANT4 Application for Tomographic Emission). Radiationto be targeted to the detection supposes gamma rays of 511 KeV energy.Calculations performed for the case of using W, Pt, Pb, In, and Al asthe material of the metal film. The calculated expression of theradiation detection efficiency y is approximated byy=(1−exp(−a*x))*exp(−b*x). In this case, the thickness of the metal filmis x (cm), the radiation absorption coefficient of metal is a (1/cm),and the average time to produce recoil electrons in the metal is b (cm).

As seen from the graph, detection efficiency y have maximum value ymaxin the graph. Therefore, it is possible to calculate the values x1 andx2 satisfying the formula: x1=x2=ymax/2(x1<x2). For example, When usingW as a metal film material, ymax is equals to 0.0325, and then ½*ymax isequals to 0.01625. then x1 and X2 are calculated 0.005 cm and 0.050 cmrespectively. From this result, by setting the thickness x of the metalfilm between 0.005 cm to 0.050 cm, i.e. by setting within half width,detection efficiency is value not less than 0.015. It shows that thedetection efficiency can be improved significantly compared toconventional radiation detecting device.

And in the case of other metals (Pb, Pt, and In), the radiationdetection efficiency can be improved, by setting the thickness in thehalf width of the maximum value in the plotting of the radiationdetection efficiency versus thickness. As a result of the calculationabout metals not shown in FIG. 2, we found that the improvement of theradiation detection efficiency is effective specifically in the case ofW, Pt, In, Fe, Pb and Cu.

(Structure of the Radiation Detecting Element)

A radiation detecting element shown in FIG. 1 is the element havingSchottky diode structure to describe below. A metal film is arranged inthe radiation incident side on an intrinsic semiconductor substrate, andN-type semiconductor region or P-type semiconductor region is arrangedin the opposite side. A depletion layer formed in the substrate can bethickened, by utilizing an intrinsic semiconductor substrate. Becauserecoil electrons generated by the incident radiation go through theintrinsic semiconductor region and the depletion layer and generate manyholes and electrons, response speed and sensitivity of the radiationdetecting can be improved. Therefore, regardless of the metal filmthickness, it shows the detection characteristics superior toconventional radiation detecting elements not utilizing the intrinsicsemiconductor substrate. And so it is possible to improve thecharacteristics of the radiation detector by setting the thickness ofthe metal to an appropriate range.

(Another Structure of the Radiation Detecting Elements)

The structure of the radiation detecting element is also possible withthe structure of the Schottky diode structure without intrinsicsemiconductor substrate. FIG. 3( a) shows a cross-sectional view ofSchottky diode structure radiation detecting element without intrinsicsemiconductor substrate. By not utilizing InCdTe but utilizing metal ofW, Pt, In, Fe, Pb, or Cu etc. As Schottky electrode, and by optimizingthe thickness of the film, the detection efficiency can be improved. Itis possible for the metal film suppressing the radiation energy and theSchottky electrode to be produced as a combined utilization film (singlelayer film). Also as shown in FIG. 3( b), it is possible for the metalfilm and the Schottky electrode to be produced as a laminated filmcomprised from another material. In the case of the single layer film,it has the merit that producing process becomes easy. And, in the caseof the laminated film, it has the merit that device design becomes easy,because the material appropriate for the radiation blocking capabilitycan be utilized as the metal film, and the material having highworkability and able to form appropriate Schottky junction for thesemiconductor material can be utilized as the Schottky electrode.

In FIG. 1, and FIG. 3 (a) it is described that the N-type semiconductorsubstrate is utilized, and the detecting element is the diode whoseforward direction is from the radiation incident side to the Ohmicelectrode. It is also possible to utilize P-type semiconductorsubstrate, and diode whose forward direction is from the Ohmic electrodeto the radiation incident side. In this case, the detecting efficiencycan also be improved by forming the metal film in the radiation incidentside, and by controlling the thickness of the film in an adequate range.

(Detecting Elements Except Schottky Diode)

In addition to this Schottky diode, a PN junction diode, PIN diode, SITand other device that can be used as radiation detecting devices inaccordance with the present invention.

FIG. 3( b) is a schematic of the cross-sectional view of radiationdetecting elements of PIN diode structure according to the presentinvention. In the radiation detecting element shown in FIG. 3( b), PINdiode is formed, for example, by arranging P-type semiconductor layer 19on one surface of a intrinsic semiconductor substrate comprised from Sietc., and by arranging N-type semiconductor layer 19 on the othersurface. The metal film 20 comprised from tungsten (W) is formed on theP-type semiconductor layer 19, which is the radiation incident side. AOhmic electrode 16 comprised from aluminum (Al) etc. is formed on N-typesemiconductor layer 17. Because the electrodes 20 are comprise frommetal such as tungsten (W) etc., it is possible to improve the radiationdetection efficiency by setting the thickness in the appropriate range.And about electrode 20, it is also possible to use laminated film ofalloy of W/Pt, W/Pb, W/Fb, W/In, W/Cu or alloys comprised from these,not using a single-layer metal film as shown in FIG. 3( b).

FIG. 3( c) shows the cross-sectional view of the radiation detectingelement of SIT (Static Induction Transistor) type according to thepresent invention. Radiation detecting element shown in FIG. 3( c) isformed by diffusion or ion implantation etc. of a P-type gate region 24on the surface of a N-type intrinsic semiconductor substrate 23, forexample comprised from N-type Si. Also, on the surface of the intrinsicsemiconductor substrate 23 surrounded by P-type gate region 24, a sourceelectrode 25 is formed by metal film, for example comprised from W. Onthe intrinsic semiconductor substrate 23, an insulation film 26 isdeposited, and also a cover film 27 comprised from metal if formed inorder to prevent radiation incidence. On the back of the intrinsicsemiconductor substrate 23, a drain electrode 21 and an N+-typesemiconductor region 22 for Ohmic contact are formed. The gate voltageapplied to the gate region 24 is set so that PN junction between thegate region 24 and the intrinsic semiconductor substrate 23 becomesreverse bias. And a depletion layer is formed near the gate region 24.When the radiation is entered through the source electrode 25, theradiation incidence is detected by detecting change of the currentflowing through drain electrode 21. Because the source electrode 25 iscomprised from metal such as tungsten, it is possible to improve theradiation detection efficiency by setting the thickness of the metal inthe appropriate range. About the source electrode 25, laminated filmcomprised from two or more metals or metal film obtained by diffusingN-type impurities on the N+ region in high concentration can be formed,in addition to the single layer film shown in FIG. 3( c).

In accordance with the present invention, each device structure ofradiation detecting elements detects radiation, placing pluralelectrodes in the direction perpendicular to the incidence direction ofthe radiation. Thus, efficiency of collecting carrier generated by theradiation incidence is high compared with the case that pluralelectrodes are arranged in the horizontally direction with the radiationincidence.

(Material of Metal Film and Semiconductor Layer)

In accordance with the present invention, W, Pt, In, Fe, Pb, Cu or thesealloys can be used as material of metal film of radiation detectingelement. Also W/Pt, W/Pb, W/Fb, W/In, W/Cu or these alloys can be usedas laminated film. W is specifically preferred as the metallic materialof the metal film. Because W has high radiation absorption efficiency,and many electrons are generated by the radiation incidence even if thefilm is thin, it has an effect with detecting element's small lightweighting.

And in the case of forming laminated film comprised from metalfilm/Schottky electrode as the radiation detecting element, for example,W/Pt, W/Pb, or W/Pb alloy can be utilized as the material of the film.Because W is difficult to process due to its high melting point, itbecomes easy to manufacture the radiation detecting element by formingmaterial film of Pt, Pb, or Pb alloy (solder) etc. on the surface bondedto semiconductor substrate.

Also in accordance with the present invention, Si, Ge, ZnO, CdTe,CdZnTe, SiC, GaN or GaAs etc can be used as the material of thesemiconductor substrate of the radiation detecting element. For exampleZnO can be processed into a film shape, it is possible to form a curvedplate-like not only to form detecting devices.

<Radiation Detecting Device>

(Radiation Detector Array)

FIG. 4( a) shows a plane view of the radiation detector array elementthat makes up the radiation detecting device of the present invention.Radiation detector array is that plural radiation detecting elements arearranged on the surface of two-dimensional (2D) plane or curvedsubstrate. FIG. 4( a) shows a radiation detecting elements of 8×8=64spaced from each other on insulating crystal 31. The Schottky electrodeon the radiation incident side is utilized as a common electrode. FIG.4( a) shows a plane view from the Ohmic electrode side opposite to theSchottky electrode, and the pad 33 corresponding to the number ofradiation detecting elements are placed around the substrate. The eachpad 33 and the each ohmic electrode are electrically connected by a bondwires 34.

Detection unit of radiation detecting device in the usual manner has thestructure that the plural radiation detector array is arranged aroundthe analyte in the shape of a ring. By simultaneously detecting gammarays emitted in opposite directions from the analyte by two radiationdetector array arranged oppositely, the location and direction where theradiation is emitted can be identified. And, by laminating the pluralradiation detector arrays arranged in a ring as shown in FIG. 4( b), apipe-shaped detecting device can be constructed (FIG. 4( c)). Theanalyte like human body is entered in the pipe-shaped detecting device,and emitting radiation is measured. It is possible to displaythree-dimensional (3D) image density of the radiopharmaceuticals, byprocessing the detected signals.

FIGS. 4( d)-4(g) shows the external views of the other embodiment of theradiation detector array according to the present invention. In thesystem that the each pad arranged on the circumference of the substrateand the each detecting element's electrode are connected by the bondingwire as shown in FIG. 4( a), when N is increased, bonding becomes verydifficult because the number of the N×N array's detecting elementsincreases by square. Other embodiment shown in FIGS. 4( d)-4(g) is theexample of array structure that is available when the high-densityimplementation is necessary, for example, in the case that there aremany detecting elements, or that detecting elements are microscopic.

FIG. 4( d) is the plane view of an electrode stiffening plate comprisedfrom insulator, plural openings, on which detecting elements arearranged, are formed on the substrate 81. FIG. 4( e) is the externalview of the detecting element. The detecting element is a Schottky diodecomprised from an electrode 82, a semiconductor 83, and an Ohmicelectrode 84. The electrode is comprised from metal W, In or Pb etc.,and is the flat-board shaped electrode. On the electrode 82, thesemiconductor 83 is formed. The semiconductor 83 is, for examplecylindrical, and comprised from intrinsic Si/N-type Si etc. At theinterface between the electrode 82 and the semiconductor 83, Schottkyjunction is formed. The Ohmic electrode 48 is the metal electrode, andfor example pin electrode. The ohmic electrode 84 is arranged on thesemiconductor 83, contacted through N+ region formed in the interface soas to form ohmic junction.

FIG. 4( f) is the perspective view of the radiation detector array whicharranges plural radiation detecting elements on the substrate 85. FIG.4( g) is its side view. The Schottky electrodes 83 of the pluraldetecting elements are the common electrode. On the other hand, thesemiconductor 83 and the ohmic electrode 84 are arranged separately, andelectrically separated. As shown in FIG. 4( g), the substrate 85 isformed by laminating a Schottky electrode 89 and a stiffening plate 88.When gamma rays are incident to the detecting element, electric currentflows between the electrode 89 and the Ohmic electrode 87, and thedetection of the gamma rays' incidence is executed.

But not shown in Figure, a shielding film or a cap comprised from metalW etc. may be arranged on the Ohmic electrode side of the radiationdetecting element. In this case, the incidence of the radiation from theOhmic electrode can be prevented, and then erroneous detection of theradiation can be prevented.

The radiation detector arrays shown in FIG. 4( f) may be directlyinstalled on the substrate by pins arranged in the shape of thepinholders, or may be connected to interface like connector. In thiscase, the detecting signal can be sent to detecting circuit and controlcircuit, and then high-density implementation is able.

(Radiation Detection Signal Processing Circuit)

The output of the radiation detecting element is analyzed by utilizing asignal processing circuit. FIG. 5 is the block diagram of the signalprocessing circuit. Output signal of the each radiation detectingelement is directly connected to the signal processing circuit. Then,different from signal processing circuit like CCD, output signal of theeach element can be simultaneously processed, and then highly preciseimage analysis is able.

In FIG. 5, a circuit block described with a radiation detecting diode 41and a reset circuit 42 shows circuit of the respective detectingelement. For example, the detecting element comprised from the Schottkydiode can be shown as equivalent circuit comprised from diode andparasitism resistance as shown in the figure. When the radiation isentered to the detecting element 41, carrier comprised from electron andpositive hole is generated, new carrier is generated by the collisionbetween the carrier and semiconductor crystal lattice, and then electriccurrent continuously flows in the diode by one time of the radiationintense. In order to distinguishably detect each radiation incidence,the reset circuit 42 is executed when the electric current is detected,and the electric current which flows in the diode is shut downed.

And, the whole radiation detecting device is shown by circuit blockcomprised from radiation detecting part 43, reset circuit part 44, ORlogical circuit 45, transfer unit 46 and computer 47. The embodimentshown in the figure is the block diagram in which number of theradiation detecting elements is 8×8=64. The transfer unit 46 transfersthe signal detected by the radiation detecting part 43 to the computer47. The OR logical circuit controls not to transfer data when thedetected electric current does not flow through all of the detectingelements. Signal processing by the computer 47 is executed for thedetecting signal transferred by the transfer unit 46, and is shown, forexample as a three-dimensional (3D) density image of theradiopharmaceutical to a display part which is not shown.

(Application of the Radiation Detecting Device)

Measurement subject of the present invention's radiation detectingdevice is not only gamma rays which is the measurement subject of thePET device, but also other radiation detecting, for example alpha rays,beta rays, neutron rays or X-rays etc. For other radiation detecting,high detecting efficiency can be obtained. As the applications of theradiation detecting device, the present application has high efficiencynot only for the PET device, but also extensive field, for exampleradiation detecting element, image diagnosis device and imaging deviceetc. utilized in the field of nuclear medicine, atomic energy, astronomyand cosmic-ray physics.

EXAMPLE

The present invention is described with example below, but the presentinvention is not restricted to this.

FIG. 6( a) to 6(c) show the simulation results of the radiationdetecting sensitivity. In the simulation, following radiation detectingmodel, in which three kind of experiment materials are placed 10 mm awayfrom a source 18F (isotope 18 fluorine, injection for PET) of diameter 4mm and thickness 0.2 mm, was utilized. 74,000,000 (event number)annihilation gamma rays were generated from the source, the number ofthe gamma arrays collided to the experiment material was calculated byMonte Carlo method, and the detected number (count number) of gammaarrays having respective energy values was calculated by Monte Carlomethod. Histogram shown in FIG. 6 was derived, by memorizing theradiation detecting sensitivity (count number) to the vertical line,memorizing the energy value to the horizontal axis, and dividing energyvalue 0 to 600 kev in 128. The detecting sensitivity at energy value 511kev was shown with bold-faced number at the top right corner of thegraph.

FIG. 6( a): (Conventional example) Only CdTe (thickness 1 mm), Detectingsensitivity 18574 FIG. 6( b): (Present invention) W (thickness 0.1mm)/CdTe (thickness 1 mm), Detecting sensitivity 20058 (increased about10% compared to the conventional example) FIG. 6( c): (Presentinvention) Pb (thickness 0.1 mm)/CdTe (thickness 1 mm)/Pb (thickness 1mm), Detecting sensitivity 21116 (increased about 20% compared to theconventional example) From the above simulation results, it wasindicated that the detecting sensitivity is largely increased with 10%to 20% by utilizing the present invention's radiation detecting element.

And if the detecting sensitivity is increased 20% for one radiationdetecting element, the detecting efficiency of the PET is calculatedsquare of 1.2, and the detecting sensitivity is increased 44%, becausethe radiation detecting elements are arranged both sides of theradiation source in the case of PET.

INDUSTRIAL APPLICABILITY

As described above, the present invention arranges the metal film on theradiation incident side of the radiation detecting element so as tocause the energy loss of the incident radiation. Then the carriergeneration efficiency is improved, and the radiation detectionefficiency can be improved.

The invention claimed is:
 1. A radiation detecting element, comprisedof: an n-type semiconductor substrate or a p-type semiconductorsubstrate; and an electrode arranged in the radiation incident side onthe n-type semiconductor substrate or the p-type semiconductorsubstrate, wherein the electrode is one of a metal film or a laminatedfilm including the metal film, and wherein, with x (cm) defined as athickness of the metal film, a (1/cm) defined as a mass attenuation ofthe metal of the metal film, and b (cm) defined as a mean range of agenerated recoil electron in the metal of the metal film, the thicknessx (cm) is arranged in a range of a half-width of an approximatedradiation detection efficiency of the radiation detecting elementdefined as y=(1−exp(−a*x))*exp(−b*x).
 2. The radiation detecting elementaccording to claim 1, wherein an intrinsic semiconductor substrate isarranged between the n-type semiconductor substrate or the p-typesemiconductor substrate and the electrode.
 3. The radiation detectingelement according to claim 1, wherein the radiation is gamma ray orX-ray.
 4. The radiation detecting element according to claim 1, whereinthe metal of the metal film is selected from the group consisting of W,Pt, In, Fe, Pb, Cu, and an alloy of any of W, Pt, In, Fe, Pb, and Cu. 5.The radiation detecting element according to claim 1, wherein thelaminated film is formed of metals selected from the group consisting ofW/Pt, W/Fe, W/In, W/Cu, and alloys of any of W/Pt, W/Fe, W/In, and W/Cu.6. A radiation detecting device which detects radiation by utilizing theradiation detecting element according to claim
 1. 7. A PET device whichdetects radiation by utilizing the radiation detecting element accordingto claim
 1. 8. The radiation detecting element according to claim 3,wherein the metal of the metal film is selected from the groupconsisting of W, Pt, In, Fe, Pb, Cu, and an alloy of any of W, Pt, In,Fe, Pb, and Cu.
 9. The radiation detecting element according to claim 3,wherein the laminated film is formed of metals selected from the groupconsisting of W/Pt, W/Fe, W/In, W/Cu, and alloys of any of W/Pt, W/Fe,W/In, and W/Cu.
 10. A radiation detecting device which detects radiationby utilizing the radiation detecting element according to claim 3.